ATOMIC REFRACTION 83 



But even for univalent elements, such as the halogens, 

 where there is no possibility of different linkages, small but 

 regular differences are observed, thus : 



A 



Chlorine 5-62 to 6.37 mean 6-014 



Bromine 8-64 9.06 8-863 



Iodine 13-58 13.91 13-808 



again in fair agreement with the values given on p. 81, 

 for the free elements. The additive character is thus far- 

 reaching in the region of optics. Finally we should have 

 to discuss the connexion with the relations of volume were 

 it not that heat of formation causes the essential moment 

 to lie in constitutive influences. 



E. Dielectric Constant. 



Just as light travels fastest in a vacuum, so the attract- 

 ing and repelling action of electric charges is greatest (a) if 

 vacuum separates them, whilst in a space partly filled with 



matter the forces are diminished to a fraction, say -^ . The 



quantity K in this expression is the so-called dielectric 

 constant, and the diminution of electric action may be 

 explained by assuming that matter is conducting for elec- 

 tricity, vacuum not. This connexion between electricity 

 and light, which finds its mathematical expression in the 

 electromagnetic theory of light, leads by the help of that 

 theory to a simple relation between the dielectric constant 

 and the refractive index for infinitely long wave-length 

 (N) viz. : 



K = N 2 . 



The latter may be calculated for substances of normal 

 dispersion from Cauchy's formula 1 , and the relation thus 

 brought out numerically. The numbers are for 18 and 

 atmospheric pressure : 



1 Landolt and Jahn, Zeitschr. f. Phys. Chem. 10. 300. 

 F 2 



